1. Field of the Invention
The present invention relates to a flow rate detecting element and a flow rate sensor for measuring an amount of inlet air of an internal combustion engine, for example, and more particularly, to a flow rate detecting element and a flow rate sensor having a heating unit for measuring a flow velocity or a flow rate of a fluid, on the basis of the heat transfer phenomenon to the fluid, from the heating unit or a portion heated by the heating unit.
2. Description of the Related Art
FIGS. 22 and 23 are a sectional view and a plan view, respectively, illustrating a conventional thermosensitive flow rate detecting element disclosed, for example, in Japanese Patent Publication No. H05-7,659.
In FIGS. 22 and 23, an insulating supporting film 2 is formed on a surface of a flat substrate 1. A lattice-shaped heating resistance element 4 is formed on the supporting film 2. Lattice-shaped temperature detecting resistance elements 5 and 6 are formed symmetrically on the supporting film 2 with the heating element 4 in between. Further, an insulating protecting film 3 is formed on the supporting film 2 so as to cover the heating resistance element 4 and the pair of temperature detecting resistance elements 5 and 6. In these drawings, the heating resistance element 4 and the pair of temperature detecting resistance elements 5 and 6, wrapped by the supporting film 2 and the protecting film 3, form a sensor section 14.
An air space 9 is provided below the sensor section 14 of the flat substrate 1. The air space 9 is formed by removing a part of the flat substrate 1 from an opening 8 by means of an etching solution not damaging the supporting film 2 and the protecting film 3. Thus, the sensor section 14 forms a bridge supported by the flat substrate 1 at the both ends thereof, and is in a non-contact state with the flat substrate 1.
A lattice-shaped comparative resistance element 7 is formed on the supporting film 2 at a position far from the sensor section 14, and covered by the protecting film 3.
The flat substrate 1 is made of a semiconductor, and particularly, of silicon which permits application of a highly precise etching technology and gives a high chip productivity. The supporting film 2 and the protecting film 3 are made of silicon nitride which is a very excellent thermal insulator. Further, the heating resistance element 4, the temperature detecting resistance elements 5 and 6 and the comparative resistance element 7 are made of platinum.
In the conventional flow detecting element having the configuration as described above, heating current fed to the heating resistance element 4 is controlled by a control circuit not shown so that temperature of the heating resistance element 4 is kept higher by 200.degree. C. than temperature of the flat substrate 1 detected by the comparative resistance element 7. Because of the presence of the air space 9 below the sensor section 14, heat produced in the heating resistance element 4 is not conducted to the comparative resistance element 7. Temperature detected at the comparative resistance element 7 is therefore substantially equal to the open air temperature.
Heat produced at the heating resistance element 4 is conducted through the supporting film 2 and the protecting film 3 and transmitted to the temperature detecting resistance elements 5 and 6. Further, the heat is transmitted through air surrounding the sensor section 14 to the temperature detecting resistance elements 5 and 6. Since the sensor section 14 is symmetrically configured relative to the center of the heating resistance element 4 as shown in FIG. 23, there is produced no difference in temperature between the pair of temperature detecting resistance elements 5 and 6 in the absence of an air flow. In the presence of an air flow, the temperature detecting resistance element in the upstream is cooled, and the temperature detecting resistance element in the downstream is heated by the heat transmitted by air from the upstream, not cooled so much as the upstream temperature detecting resistance element. When an air flow is produced in the arrow 10 direction in FIG. 23, for example, temperature of the temperature detecting resistance element 6 is higher than that of the temperature detecting resistance element 5. A higher flow velocity leads to a larger difference in temperature between these temperature detecting resistance elements 5 and 6.
Because temperature of the temperature detecting resistance elements 5 and 6 is expressed in terms of a resistance value, the flow velocity can be measured by detecting the difference in resistance value between the temperature detecting resistance elements 5 and 6.
If the air flow direction is counter to the arrow 10 direction, temperature of the temperature detecting resistance element 6 is lower than that of the temperature detecting resistance element 5. Therefore, it is also possible to detect the flow direction of the fluid.
The conventional thermosensitive flow rate detecting element shown in FIGS. 22 and 23 is of the bridge type, and apart from this, the diaphragm type is available as a thermosensitive flow rate detecting element.
A conventional diaphragm type thermosensitive flow rate detecting element is illustrated in FIGS. 24 and 25.
In this diaphragm type thermosensitive flow rate detecting element, a recess 13 is formed by removing a part of the flat substrate 1 by etching or the like from the surface opposite to that having the sensor section 14 formed thereon. The sensor section 14 is held by the flat substrate 1 over the entire periphery thereof to form a diaphragm, in a non-contact state with the flat substrate 1.
This diaphragm type thermosensitive flow rate detecting element, being supported by the flat substrate 1 over the entire periphery, can provide a higher strength, but an inferior response, than a bridge type thermosensitive flow rate detecting element. The principle of detection of the flow velocity of a fluid is the same as in the bridge type.
The conventional thermosensitive flow rate detecting elements have the configuration as described above. When the flow rate or the flow velocity of a measured fluid changes, therefore, there occurs a delay dependent on the thermal conductivity and the heat capacity of the supporting film 2, the protecting film 3, and the individual resistance elements 4, 5 and 6 in temperature of the heating resistance element 4 and the temperature detecting resistance elements 5 and 6. While the heating resistance element 4 is temperature-controlled, the temperature detecting resistance elements 5 and 6 provided on the both sides of the heating resistance element 4 are not temperature-controlled. A period of time is therefore required before a prescribed temperature permitting accurate detection of the flow rate and the flow velocity of the measured fluid after change in temperature.
Therefore, when the flow rate or the flow velocity of a measured fluid continues always to change, temperature of the temperature detecting resistance elements 5 and 6 never accurately represents the instantaneous flow rate or flow velocity. A larger change in flow velocity of the measured fluid per unit time leads to a more difficult detection of an accurate instantaneous flow rate or flow velocity, i.e., to a poorer response as a flow rate detecting element.
Reduction of thickness of the supporting film 2, the protecting film 3 or the resistance element 4, 5 or 6 tends to solve such an inconvenience. This however causes a serious decrease in strength of the sensor section 14 of the bridge structure or the diaphragm structure, and hence poses a problem of a lower reliability as a flow rate detecting element.
For example, when measuring the amount of inlet air of an internal combustion engine, the amount of inlet air becomes a pulsation flow responding to the number of revolutions, which results in a very large range of fluctuation of flow rate in a high-load region and a high speed of fluctuation of flow rate in a high rotation region. A highly responsive flow rate detecting element is therefore demanded. Further, since the maximum flow velocity of inlet air almost reaches 200 m/s, a flow rate detecting element having a prescribed strength is required.
In the conventional thermosensitive flow rate detecting element, as described above, improvement of response requires a decrease in strength, and this poses the problem of very difficult design of an element suitable for measurement of the amount of inlet air of an internal combustion engine.